Jet fuel compositions



United States Patent F 3,095,287 JET FUEL COMPOSITIONS Thomas H. Coflield, Farmington, and Allen H. Filbey, Walled Lake, Mich., assignors to Ethyl Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Aug. 28, 1961, Ser. No. 134,141 Claims. (Cl. 44-78) This invention relates to new jet fuel compositions characterized by high thermal stability.

Fuel temperatures in modern jet aircraft power plants are becoming so high that harmful deposits are formed in the pre-comhustion phase of the fuel system. Contributing to this has been the use of the fuel as a heat sink to aid in lubricating oil cooling, which has increased fuel temperatures to the point where deposits are so severe that they interfere. with normal fuel combustion as well as lubricating oil temperature control. The jet fuel thermal stability problem is so serious that it can eventually lead to engine failure of the turbine section due to uneven temperature patterns. In fact, it is considered the outstanding problem in jet fuels at the present time.

Prior investigators have found that conventional gasoline antioxidants are incapable of overcoming this oppressive problem. The art is replete with reports by eminent investigators which are universally to the effect that conventional antioxidants do not overcome the jet fuel thermal stability problem. For example, it has been stated that neither 4methyl-2,6- di-tert-butyl-phenol nor N,N-di-sec-butyl-p-phenylenediamine improves the high temperature stability of jet fuels. In fact, some gasoline antioxidants have been shown to be deleterious in that they increase the severity of the problem. Consequently, the experts in the field have turned their attention to other types of additives-Le, materials which are not antioxidants. One approach has been the use of dispersants in an attempt to keep the deposits suspended in the fuel and thereby prevent them from adhering to critical engine surfaces. However, this approach has not proved satisfactory because the deterioration of the fuel does occur under jet engine operating conditions and little, if any, improvement in engine performance has as yet been attained. Another approach has been the use of various jet fuel treating procedures. However, these are unsatisfactory because they are expensive and complicated, and, in most cases, little improvement is achieved. Special fuel blending procedures have also been suggested but found totally impractical. Thus, in general, all approaches to the solution of this substantial problem have thus far been unsuccessful.

An object of this invention is to alleviate the thermal stability problems in jet fuels. Another object is to provide new jet fuel compositions which are characterized by a high degree of thermal stability. A further object is to overcome the thermal stability problems in jet fuels in a simple and inexpensive manner. A still further object is to provide processes of inhibiting deterioration of jet fuel normally tending to occur at elevated temperatures below the cracking temperatures of the fuel. Other objects will be apparent from the ensuing description.

The unexpected and unprecedented discovery has been made that the above and other objects of this invention are achieved by providing jet fuel containing from about 0.01 to about 0.1 percent by weight of a 3,3',5,5-tetraalkyl-4-4-dihydroxydiphenyl. The thermal stabilizers of this invention exhibit the unique property of greatly improving the thermal stability of jet fuels and this effectiveness is independent of the hydrocarbon types from 3,095,287 Patented June 25, 1963 which the jet fuel has been prepared. Thus, the present invention affords extreme protection against thermal instability of all present-day jet fuels.

The jet fuel additives of this invention directly attack and overcome the thermal instability problem at its source. They icon-fer high thermal stability upon the finished fuels of this invention so that the fuels strongly resist thermally-induced degradation. Thus, markedly reduced is the amount of insoluble thermal decomposition products which heretofore deposited to plug orifices in the fuel system, to distort fuel flow and thus impair flame pattern, and to foul surfaces. Furthermore, the additives of this invention do not introduce secondary problems in use, such as jet fuel foaming at high altitudes, emulsification difficulties, interference with low-temperature flows, and the like. At the same time all of these highly important and unique advantages are achieved in a simple manner and at very low cost. Hence, the present invention represents a substantial contribution to the jet fuel art.

It is known that conventional jet fuel normally tends to deteriorate when subjected to the condition of elevated temperatures below the cracking temperature of the fuel, i.e., temperatures in the range of about 300 to about 500 F. Hence, another part of this invention is the process of inhibiting such deterioration which comprises subjecting a jet fuel containing from about 0.01 to 0.1 percent by weight of a 3,3,5,5-tetraalkyl-4,4-dihydroxydiphenyl to said condition. Thus, greatly enhanced thermal stability of jet fuel is achieved by blending with a jet fuel from about 0.01 to about 0.1 percent by weight of a 3,3, 5,5-tetra-a1kyl-4,4'-dihydroxydiphenyl and subjecting the resulting fuel to the above condition.

The 3,3,5,5'-tetraalkyl-4,4'-dihydroxydiphenyl jet fuel additives have the formula wherein R and R are alkyl groups which preferably contain up to about 12 carbon atoms.

Preferred jet fuel additives of this invention are 3,3- 5,5-tetraalkyl-4,4-dihydroxydiphenyls in which each phenyl group carries at least one secondary or tertiary alkyl groupi.e., an alkyl group which is branched on the alpha carbon atom. These preferred additives are particularly effective in improving the high temperature stability characteristics of jet fuels.

Particularly preferred as jet fuel additives are 3,3',5, 5-tetra-tert-butyl-4,4' dihydroxydiphenyl; 3,3'-dimethyl-5, 5'-di-ter-t butyl-4,4'-dihydroxydiphenyl; 3,3,5,5-tetraisopropyl-4,4'-dihydroxydiphenyl; and 3,3'-dimethyl-5,5'-diisopropyl-4,4-dihydroxydiphenyl. These compounds not only possess the excellent attributes of the preferred class of additives described above, but are easily made from 2,6-dialkyl phenols which themselves are readily prepared. Furthermore, these 2,6-dialkyl phenols used as starting materials and the particularly preferred thermal stabilizers of this invention are inexpensive and prepared in high yields and purity.

The jet fuels whose thermal stability is greatly improved pursuant to this invention are principally hydrocarbon fuels which are heavier than gasoline, i.e., distilled liquid hydrocarbon fuels having a higher endpoint than gasoline. In general, the jet fuels can be comprised of distillate fuels and naphthas and blends of the above, including blends with lighter hydrocarbon fractions, so long as the endpoint of the final jet fuel is at least 435 F. and preferably greater than 480 F. It will be understood, however, that the jet fuels which are employed according to this invention can contain certain other ingredients, such as alcohols or the like, provided the resulting fuel blend meets the specifications imposed upon jet fuels.

Another characteristic of the jet fuels employed according to this invention is that they have an API gravity of from 35-50". This characteristic is very important. Hydrocarbons having lower API gravities would not be suitable as jet fuels because of undesirable viscosity and flow characteristics attendant with such low API gravities. These other hydrocarbons have utility in other areas, such as lubricating oils and turbine oils. The following table, taken from page 59 of the text Petroleum Refinery Engineering, by W. L. Nelson, 4th Edition (1958), McGraw-Hill Book Co., Inc, New York, illustrates the utilities of various hydrocarbons having API gravitics less than 32.

TABLE I.--API GRAVITIES OF LUBRICATING OILS Engine and machine, heaviest Heavy machinery Cold test, light Refrigeration, etc. do

Cold test, heavy 25-28 Cylinder oils, unfiltered:

Light mineral I 25-28 Heavy mineral Englne r compressor eyl- -20 inders. Light compounde -28 Heavy compounded 20-26 Marine engine, mineral Marine engines 23-28 Marine engine, compound do 23-28 Turbine oil, light Steam turbines, dynamos, 29-31 high speed, etc. Turbine oil, medium 27-30 Turbine oil, heavie t 26-28 Transformer oil Electrical transformers 28-30 Black oil, summer Rough slow speed bearings, 20-25 erushers, etc. Black oil, winter 20-27 White oil Food manufacturing, textiles, 29-32 paper, etc.

Typical jet fuels improved according to this invention include JP-3, a mixture of about 70 percent gasoline and 30 percent light distillate having a 90 percent evaporated point of 470 F.; JP-4, a mixture of about 65 percent gasoline and percent light distillate-a fuel especially designed for high altitude performance; JP-5, an especially fractionated kerosene; high flash point-low freezing point kerosene, etc.

The following are specifications of typical liquid hydrocarbon jet fuels of this invention:

FuelA FuelB FuelC FuelD FuelE FuelF (JP-3) (JP-4) (JP-5) (J P-4) (JP-4 (Keroreieree) some) 10 Eva orated r i .1. 160 220 395 221 330 90% Evaporated,

F 470 470 480 379 460 480 Endpoint F 600 550 550 480 516 Gra\'ity, API 5o 35 47.3 48.5 43 Existent Gum,

rug/100 ml. max. 7 7 7 1.0 1.4 1. 7 Potential Gum,

mg/100 ml. max. 14 14 14 1.0 9. 6 Reid Vapor Pressure, p.s.i 7.0 3.0 Aromatics, vol.

percent 25.0 25.0 25.0 12.5 14.6 14.3 Olefins, vol. percent 5.0 5.0 5.0 0.3 1.2

The following examples illustrate various specific embodiments of this invention.

Example I To 100,000 parts of Fuel A is added With stirring 10 parts (0.01 percent) of 3,3,5,5-tetramethyl-4,4-dihydroxydiphenyl dissolved in 200 parts of ethanol. The resulting fuel is found to possess improved thermal stability characteristics.

Example 11 To 100,000 parts of Fuel B is added 100 parts (0.1 percent) of 3,3'-dimethyl-5,5'-diethyl-4,4'-dihydroxydiphenyl dissolved in 500 parts of methanol. The resulting fuel possesses improved thermal stability properties.

Example III With 100,000 parts of Fuel C is blended 50 parts (0.05 percent) of 3,3-dimethyl-5,5'-diisopropyl-4,4-dihydroxydiphenyl. The resulting fuel blend possesses vastly superior thermal stability characteristics.

Example IV To 100,000 parts of Fuel D is added 40 parts (0.4 percent) of 3,3'-dirnethyl-5,5-di-tert-butyl-4,4-dihydroxydiphenyl dissolved in 400 parts of toluene. The resulting fuel blend is found to possess greatly superior thermal stability characteristics.

Example V With 100,000 parts of Fuel E is blended parts (0.08 percent) of 3,3,5,5'-tetra-tert-butyl-4,4'-dihydroxydiphenyl. The resulting fuel blend possesses greatly enhanced thermal stability properties.

Example VI To 100,000 parts of Fuel F is added 20 parts (0.02 percent) of 3,3',5,5'-tetraisopropyl-4,4-dihydroxydiphenyl dissolved in 250 parts of isopropanol. The resulting fuel blend is found to possess greatly enhanced thermal stability properties.

Example VII Fuel C is blended with a lighter hydrocarbon fraction to give a final jet fuel having an endpoint of 435 F. To 100,000 parts of the resultant fuel is added with stirring parts (0.09 percent) of 3,3',5,5'-tetra-(2-dodecyl)- 4,4-dihydroxydiphenyl. The resulting fuel possesses improved thermal stability characteristics.

Example VIII To 100,000 parts of a liquid hydrocarbon jet fuel having an endpoint of 550 F. is added 30 parts (0.03 percent) of 3,3'-diisopropyl-5,5'-di-tert-butyl-4,4'-dihydroxydiphenyl dissolved in 250 parts of mixed xylenes. The resulting jet fuel possesses superior thermal stability properties.

Example IX With 100,000 parts of Fuel A is blended 60 parts (0.06 percent) of 3,3'-dimethyl-5,5-di-tert-amyl-4,4-dihydroxydiphenyl. This fuel, after mixing, possesses greatly improved thermal stability characteristics.

Example X With 100,000 parts of Fuel C is blended 50 parts (0.05 percent) of 3,3-diethyl-5,5'-di-tert-butyl-4,4-dihydroxydiphenyl. The resulting jet fuel blend possesses superior thermal stability characteristics.

Example XII To 100,000 parts of Fuel B is added with stirring 20 parts (0.02 percent) of 3,3-dioctyl-5,5'-diisopropyl-4,4'- dihydroxydiphenyl. The resulting jet fuel is found to possess superior thermal stability characteristics.

Examples VII, VIII, IX, X, XI and XII illustrate preferred jet fuel compositions of this invention containing prefenred 3,3',5,5tetr aalkyl 4,4 dihydroxydiphenyls. Particularly preferred jet fuels of this invention are illustrated by Examples III, IV, V, and VI, since the fuels in these examples contain particularly preferred 3,3,5,5'- tetraa1kyl-4,4-dihydroxydiphenyls.

The substantial improvements resulting from the practice of this invention is illustrated by tests in an apparatus known as the Coordinating Fuel Research (CFR) Jet Fuel Coker, generally called the Erdco Rig. See Pe troleum Processing, December 1955, pages 1909-1911. In order to show the superlative results of the compositions of this invention from the standpoint of reduced preheater deposits, this CFR fuel coker was operated 'without a filter and without heat on the filter furnace. This permits all of the tests to be of equal and predetermined duration so that a direct comparison of deposits from a given quantity of fuel is provided. In one series of tests, the preheater temperature was 400 F., fuel flow was at the rate of six pounds per hour, and each individual run was carried out for a period of 150 minutes. The fuel used in these tests was a commercially available LIP-5 fuel having the following inspection data:

Gravity, API 39.0 Distillation, ASTM D86, temp, F. at percent recovered:

Start 373 5 379 386 396 405 40 414 50 422 60 432 70 444 80 459 90 480 95 497 End point 516 Recovered 98 Residue, percent 1 Loss, percent 1 Flash, PM, "F 16 4 Aniline point 132 Aniline-gravity constant 5148 Hydrocarbon type analysis, FIAM:

Aromatics, 'vol. percent 18 Oletins, vol. percent 2 Saturates, vol. percent"; 80 Viscosity,cs., at 30 F 10.68 Freezing point, F 62 Existent gum (steam jet), mg./1'00 ml 1 Potential gum, nag/100 ml 1 Total su lfur, wt. percent 0.044 Mercaptan sulfur, wt. percent 0.001 Smoke point, mm 18 Water reaction 1 Water tolerance OK, 0 ml.

To individual portions of this base fuel were added various 3,3-5,5'-tetraalkyl-4,4'-dihydrioxydiphenyls at a concentration of 80 pounds per 1000 barrels (approximately 0.025 percent by weight of additive). Each of these fuels was then subjected to the above test and the extent of deposits which formed on the preheater surfaces determined. The additive-free base fuel was also rate-d in this manner. The extent of the deposit formation on the preheater surfaces is a direct measure of the thermal stability of the fuel subjected to the test. Hence, the greater the coverage of the preheater surfaces with deposits, the greater is the thermal instability of the fuel. An additional criterion of the thermal stability of the fuels is the coloration of these deposits. If a light-colored deposit is formed, only a small amount of high temperature deterioration of the fuel has occurred. Thus, the darker the deposits, the more thermally unstable is the fuel. The results of this series of tests are shown in Table II.

TABLE II.-EFFECT OF ADDITIVES ON THE THERMAL STABILITY OF IP-S FUEL It will be seen from the data shown in Table II that all of the fuels of this invention possessed greatly improved thermal stability characteristics, especially those which contained 3,3,5,5'-tetra-tert-butyl-4,4-dihydroxydiphenyl and 3,3 dirnethyl-S,5'-'di-tert-butyl-4,4'-dihydroxyphenyl. These are particularly striking results in view of the fact that a great many commercially available gasoline antioxidants completely failed to afford any protection to this jet fuel when used at the same concentration and subjected to the same test conditions. Furthermore, this base fuel in the absence of an additive of this invention has very poor thermal stability properties. For example, when this clear base fuel was subjected'to the Erdco fuel coker test using the heated, sintered steel filter (held at 500 F.) through which the preheated fuel (preheater temperature 400 F.) was passed at a rate of six pounds per hour, a pressure drop across the filter of 25 inches of mercury occurred in only 45 minutes. A fuel that runs through this apparatus under these conditions for a full 300 minutes without causing any pressure drop is considered completely thermally stable.

To still further demonstrate the magnificant improvements in high temperature stability exhibited by jet fuels of this invention, recourse is had to the Erdco coking test using the following conditions: Unit operated at psi. with the preheater temperature .at 400 F., the filter temperature at 500 F., and a fuel flow rate of 3.2 pounds per hour. Additive-free samples of thebase fuels are likewise subjected to this test; It is found that in all instances the presence in the fuels of the 3,3',5,5'-tetraalkyl-4,4'-dihydroxydiphenyls causes a very substantial increase in the time required for a pressure drop of 25 inches of mercury across the filter to occur. Furthermore, most of the fuels of this invention do not reach this pressure drop even in the full 300 minutes. In some cases there is only an insignificant pressure drop achieved with jet fuels of this invention after 300 minutes of operation. This is borne out by the results shown in Table III which were obtained using several different jet fuel base stocks of varying thermal stabilities. In Table III jet fuel A was a straight-run kerosene base stock, B was a JP-S fuel, and C (also a IP-5 fuel) was composed by volume of 84 percent of saturates and 16 percent of aromatics.

7 TABLE III.-EFFECT OF ADDITIVES ON THE THERMAL STABILITY OF JET FUELS Addi- Pressure tive Test Drop Test Jet Additive Cone, Tirne, Across Merit No. Fuel wt. min. Filter, Rating percent Inches of Hg 1 A None 25 25 47 2 A 3,3',5,5'-tetraisc- 0.028 250 25 396 propyl-4,4-dihydroxydiphonyl. 3 A 3,3"dirnethyl-5,5- 0.055 240 0.75 720 diisopropyl-4,4- dihydroxydiphenyl. 4"... B None 64 25 122 5--. B 3,3,5,5'-tetra-tert- 0.028 185 25 315 butyl-4,4-dihydroxydiphenyl. 6 B 3,3',5,5-tetraisopro- 0.028 196 25 330 pyl-4,4-dihydroxydiphenyl. 7 None 45 25 84 8 O 3,3-dimethyl-5.5- 0.055 300 7 535 di-tert-butyl-4,4- dihydroxydiphenyl. 9 O 3,3,5,5-tetraiso- 0.055 300 3.6 594 propyl-4,4-dihydroxydiphenyl. 10 O 3,3,5.5-tetra-tert- 0.055 300 4.8 569 butyl-4.4-dihydroxydiphenyl.

1 Test discontinued because so little fuel deterioration had occured.

The data shown in Table III establish the outstanding thermal characteristics of jet fuels caused by the presence therein of the additives of this invention. It can be seen on the basis of filter plugging time, improvements of as much as 1000 percent and more were achieved by the practice of this invention. Furthermore, the various merit ratings-which are correlated to the thermal stability of the fuels in terms of filter plugging and time further show the enormous benefits achieved by blending with jet fuels the additives of this invention. In the merit rating system, a value of 900 represents perfect fuel performancei.e., no pressure drop in 300 minutes-whereas a zero rating is achieved when the fuel exhibits a pressure drop of 25 inches of mercury in zero time. Thus, the nearer the merit rating to 900, the more nearly perfect is the fuel.

Typical additives which can be used in the practice of this invention include such compounds as 3,3',5,5-tetramethyl-4,4-dihydroxydiphenyl; 3,3,5,5-tetraethyl 4,4- dihydroxydiphenyl; 3,3,5,5-tetrapropyl-4,4 dihydroxydiphenyl; 3,3,5,5'-tetrabutyl 4,4 dihydroxydiphenyl; 3,3 dimethyl 5,5 diethyl-4,4-dihydroxydiphenyl; 3,3,5,5-tetraocty1-4,4-dihydroxydiphenyl; 3,3,5,5-tetradecyl-4,4-dihydroxydiphenyl; 3,3,5,5-tetradoceyl 4,4- dihydroxydiphenyl; 3,3-diethyl-5,5-didecyl 4,4 dihydroxydiphenyl; 3,3'-dipropyl-S,5-dihexyl-4,4-dihydroxydiphenyl; and the like. Preferred additives of this invention include 3,3-diethyl-5,5'-diisopropyl-4,4'-dihydroxydiphenyl; 3,3,5,5'-tetra-sec-butyl-4,4'-dihydroxydiphenyl; 3,3',5,5'-tetra-(2-heptyl)-4,4'-'dihydroxydiphenyl; 3,3'-dibutyl-5,5-di-(3-decyl)-4,4'-dihydroxydiphenyl; 3,3 dihexyl 5,5 di(2-dodecyl) 4,4'-dihydroxydiphenyl; 3,3',5,5'-tetra (1,1,2,2 tetramethylpropyl)-4,4-dihydroxydiphenyl; 3,3'-dimethy1-5,5-tetra-tert-amyl-4,4'-dihydroxydiphenyl; 3,3 dimethyl 5,5-di(l,l,3,3-tetramethylbutyl)-4,4-dihydroxydiphenyl; and the like. As pointed out above the particularly preferred additives of this invention are 3,3,5,5-tetra-tert-butyl-4,4'-dihydroxydiphenyl; 3,3',5,5'-tetraisopropyl-4,4 dihydroxydiphenyl; 3,3-dimethyl-5,5-di-tert-butyl 4,4 dihydroxydiphenyl; and 3,3-dimethyl=5,5-diisopropyl 4,4 dihydroxydiphenyl.

The additives of this invention are prepared by oxidiz ing a 2,6-dialkyl phenol to form the corresponding 3,3',5,5'-tetraalkyldiphenoquinone which is then reduced to form the corresponding dihydroxydiphenyl compound. The oxidation is effected by using such oxidizing agents as ferric chloride, nitric acid, chromic acid, etc. Reduction is readily accomplished by the use of such reducing agents as zinc and acetic acid, stannous chloride, and the like.

The amount of the additive used in the jet fuels of this invention can range from about 0.01 to about 0.1 percent by weight. Ordinarily amounts of 0.02 to 0.06 percent are found to be very effective in present-day jet fuels. Variations from these concentration ranges are permissible. For example, in jet fuels initially possessing a fair degree of thermal stability, very small amounts of the additives are sufficient to greatly improve the thermal stability characteristics of such fuels and, in some cases, provide improved storage stability properties. On the other hand, where the jet fuel initially has a very poor thermal stability, larger amounts (about 0.05 to 0.1 percent by weight or more) can be effectively used.

In preparing the improved jet fuels of this invention, the use of solvents for the 3,3',5,5-tetraalkyl-4,4'-dihydroxydiphenyls is frequently advantageous. While the solubility of these compounds in jet fuels is sufficiently high to provide the desired concentrations, blending procedures are simplified by predissolving these thermal stabilizers in a suitable solvent. The resulting formulations can then be conveniently and readily blended with the jet fuels. Particularly suitable solvents for this purpose include benzene, toluene, xylene, acetone, methylethyl ketone, methanol, ethanol, isopropanol, methyl isobutyl carbinol, and the like. In general, ketones and alcohols containing up to about 6 carbon atoms and liquid aromatic hydrocarbons containing 6 to 18 carbon atoms are excellent solvents. Other materials that can be used in the jet fuels of this invention are anti-rust additives, dispersants, and, in general, additives which do not adversely affect the high temperature stability of the fuels.

This application is a continuation-in-part of co-pending application, Serial No. 623,541, filed November 21, 1956, now abandoned, entitled Jet Fuel Compositions, by Thomas H. Coffield and Allen H. Filbey.

We claim:

1. A thermally stabilized distilled hydrocarbon jet fuel having an endpoint of at least about 480 F. and API gravity of from 35-50 containing from about 0.01 to about 0.1 percent by weight of a 3,3,5,5-tetraalkyl-4,4- dihydroxydiphenyl in which each alkyl group contains up to about 12 carbon atoms.

2. Distilled hydrocarbon jet fuel having an endpoint of at least about 480 F. and an API gravity of from 35 50 containing from about 0.01 to about 0.1 percent by weight of a 3,3',5,5'-tetraalkyl-4,4-dihydroxydiphenyl in which each phenyl group carries at least one alkyl group which is branched on the alpha carbon atom and in which each alkyl group contains up to about 12 carbon atoms.

3. The jet fuel composition of claim 2 wherein each phenyl group of said 3,3,5,5-tetraalkyl-4,4-dihydroxydiphenyl carries at least one tertiary butyl group.

4. The jet fuel composition of claim 2 wherein said 3,3,5,5-tetraalkyl 4,4 dihydroxydiphenyl is 3,3-dimethyl-5,5-di-tert-butyl-4,4-dihydroxydiphenyl.

5. The jet fuel composition of claim 2 wherein said 3,3',5,5-tetraalkyl-4,4' dihydroxydiphenyl is 3,3,5,5- tetra-tert-butyl-4,4'-dihydroxydiphenyl.

6. A process for cooling the lubricating oil in a jet engine comprising using as a coolant for heat transfer with the lubricating oil a thermally stabilized jet fuel and consisting essentially of a distilled hydrocarbon fuel having an endpoint of at least about 480 F. and containing from about 0.01 to about 0.1 percent by weight of a 3,3,5,5'-tetraalkyl-4,4-dihydroxydiphenyl in which each alkyl group contains up to about 12 carbon atoms.

7. The process of claim 6 wherein each phenyl group of said 3,3,'5,S-tetraalkyl-4,4-dihydroxydiphenyl carries at least one alkyl group which is branched on the alpha carbon atom.

3,095,287 9 8. The process of claim 6 wherein each phenyl group of said 3,3,5,5-tetraa1kyl-4,4'-dihydroxydiphenyl carries at least one tertiary butyl group.

9. The process of claim 6 wherein said 3,3',5,5'-tetra- 2=479948 alkyl-4,4'-dihydroxydipheny1 is 3,3'-dimethy1-5,5'-di-tert- 5 2,785,188 butyl-4,4- dihydroxydiphenyl. 2,959,915

10. The pmocess of claim 6 wherein said 3,3',5,5'-tetraa1ky1-4,4'-dihydmxydipheny1 is 3,3,5,5-tetra-tert-butyl- 4,4'-dihydroxydiphenyl. 687,293

References Cited in the file of this patent UNITED STATES PATENTS Luten et a1. Aug. 23, 1949 Co e Mar. 12, 1957 Dille et a1. Nov. 15, 1960 FOREIGN PATENTS Great Britain Feb. 11, 1953 

1. A THERMALLY STABILIZED DISTILLED HYDROCARBON JET FUEL HAVING AN ENDPOINT OF AT LEAST ABOUT 480*F. AND API GRAVITY OF FROM 35-50* CONTAINING FROM ABOUT 0.01 TO ABOUT 0.1 PERCENT BY WEIGHT OF A 3,3'',5,5''-TETRAALKYL-4,4''DIHYDROXYDIPHENYL IN WHICH EACH ALKYL GROUP CONTAINS UP TO ABOUT 12 CARBON ATOMS. 